There isn't a single organization that provides completely free, full-length, official MTEL practice tests. The Massachusetts Department of Elementary and Secondary Education (DESE), which administers the MTEL, offers some sample questions and test information on their website, but not full-length practice tests for free.
Many test preparation companies offer MTEL practice tests, but these are usually part of paid subscription services or purchased individually. You might find some free *sample* questions scattered across various websites, but be cautious about their accuracy and reliability. Free resources are often limited in scope.